This invention relates to a digital transmission line fault location system which is more particularly described as a fault location system using error rates for determining the location of a fault.
Error rate measurements are made on transmission line to provide information indicating the quality of transmission such as excellent, good, fair, poor and failed. Error rate can be determined by checking parity on an in-service basis or by an out-of-service test using a pseudo-random code sequence. The in-service test requires recovery of framing at the location where the measurement is being made. This is prohibitively expensive when it is needed at each regenerator along the transmission line. The out-of-service test requires external test sets which are impractical for use at outside locations. Thus, error rate measurements are typically made on an end-to-end basis.
In many systems separate fault location circuits and equipment are being provided for locating suspected or known faults. Typically an individually assigned supervisory frequency is uniquely associated with each repeater location for purposes of polling the repeaters for fault location testing. By polling the repeaters with the uniquely assigned frequencies, it is possible to determine where a failure is located.
There are problems resulting from these testing arrangements. When the repeaters are polled with the separate circuits, it is possible to locate a complete failure in the system, but the location of a soft failure producing degraded operation is very difficult to determine. On the other hand, the known error rate monitoring arrangements, which test from end-to-end, can readily determine that a soft failure is affecting system operation but cannot readily determine where the soft failure is located.
It is desirable to develop an error rate measurement system which operates on a section-by-section basis for determining the location of any failure. Error rate measurements must be recovered from remote regenerator locations and transmitted to one of the terminals to be useful. In the prior art, facilities for these kinds of operations are not available.